Lumped-constant isolators have been used for terminal devices of a mobile communication apparatuses from early on, since they can be constructed in small size. An isolator is connected between a power amplifier and an antenna in a transmitter side of the mobile communication apparatuses, and used for such purposes as preventing back-flow of an extraneous signal into the power amplifier, stabilizing a load side impedance of the power amplifier, and so on.
There has been a strong demand for miniaturization of the isolator in itself as well as a reduction of insertion losses in order to restrain consumption of a battery, because of an expeditious tendency of downsizing the mobile communication apparatuses in recent years.
An ordinary structure of the lumped-constant isolators used widely in the latest terminal devices such as cellular phones will be described briefly by referring to FIG. 21.
Three sets of strip lines 61a, 61b and 61c, electrically insulated from one another, and multi-layered in a manner to cross with respect to each other at an angle of approximately 120 degrees, are disposed in contiguity to a ferrite substrate 62. A magnet 63 for magnetizing the ferrite substrate 62 is disposed in a position facing the ferrite substrate 62.
Capacitors 64a, 64b and 64c are connected individually in parallel with their respective strip lines 61a, 61b and 61c. Terminals of the strip lines 61a and 61b are connected to their respective input/output terminals 65a and 65b, and a terminal of the strip line 61c is connected to a termination resistor 66. The other ends of the individual strip lines are connected to a common circular ground plate 67, which is electrically connected to a ground frame 60 together with ground side electrodes of the three capacitors 64a, 64b and 64c and the resistor 66.
The ground frame 60 is then electrically connected to a lower enclosure 69 having a ground terminal for connection to the outside. In addition, an upper enclosure 68 is disposed as shown in the figure in order to house the ferrite substrate 62 and the magnet 63, and to constitute a part of a magnetic circuit in combination with the lower enclosure 69. The lower enclosure 69 is provided with windows 600 for adjusting circuit constants by trimming electrodes of the capacitors 64a, 64b and 64c. 
In the past, the three sets of strip lines 61a, 61b and 61c have all been constructed in a straight form so that each of the strip lines intersects with one another at an angle of approximately 120 degrees on the ferrite substrate 62. Although not shown in the figure, each of the strip lines is assembled with an insulation sheet inserted between them in such a manner that they do not contact electrically with each other.
As reduction in size of the portable terminal devices advance lately, there arises an increasing demand for miniaturization of the isolators, and for preventing characteristics degradation of the isolators that might otherwise occur due to the miniaturization.